Polarized Dendritic Transport and the AP-1 μ1 Clathrin Adaptor UNC-101 Localize Odorant Receptors to Olfactory Cilia

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115

The navigation of axons toward their targets is a highly dynamic and precisely regulated process during nervous system development. The molecular basis of this navigation process is only partly understood. In Caenorhabditis elegans, we isolated the RNAihypersensitive strain nre-1(hd20) lin-15b(hd126), which allows us to phenocopy axon guidance defects of known genes by feeding RNAi. We used this mutant strain to systematically screen 4,577 genes on chromosomes I and III for axon guidance phenotypes. We identified 93 genes whose down-regulation led to penetrant ventral cord fasciculation defects or motoneuron commissure outgrowth defects. These genes encode various classes of proteins, ranging from secreted or putative cell surface proteins to transcription factors controlling gene expression. A majority of the genes is evolutionary conserved and previously uncharacterized. In addition, we found axon guidance functions for known genes like pry-1, a component of the Wnt-signaling pathway, and ced-1, a receptor required for the engulfment of neurons undergoing apoptosis during development. Our screen provides insights into molecular pathways operating during the generation of neuronal circuits and provides a basis for a more detailed analysis of gene networks regulating axon navigation.Wnt ͉ neuron ͉ development ͉ axon navigation D irected outgrowth of neuronal processes like axons and dendrites reflects a complex navigational problem because of the large number of neurons involved in this process. Several conserved signaling systems have been described that influence the direction of outgrowth of navigating axons (for reviews, see refs. 1-3). Despite this progress in the identification of axon guidance signals, their small number does not match the complexity of the guidance process and the large number of neurons involved. The rather limited defects in mutants of known guidance cues indicate that a substantial fraction of axon guidance genes remains to be identified.The nematode Caenorhabditis elegans is a suitable model for such a purpose, not only because its nervous system is simple and well described, but also because large-scale screens can be performed fast and easily. C. elegans has a comparatively small nervous system, with exactly 302 neurons in the adult hermaphrodite (4, 5). Axons and dendrites are typically unbranched and grow out in a stereotypic fashion. Because it is now possible to label neurons in vivo using fluorescent proteins like GFP (6, 7), large-scale direct visual screens for axon navigation defects have become feasible. In the last few years, RNAi in C. elegans (8) has become a powerful method to identify genes controlling particular biological processes, such as longevity, fat regulation, genome stability, RNAi, or transposon silencing (9). Unfortunately, RNAi, by feeding, does not function efficiently in the nervous system (10), effectively preventing use of this tool for the molecular analysis of axon guidance. Here we report the isolation of a strain in C. elegans that shows enhanced ...

Section: Resultsmentioning

confidence: 99%

“…Unc-101 encodes a homolog of the medium chain of the clathrin adaptor complex AP1 (9,27). Trafficking of olfactory receptors depends on unc-101 (12), suggesting that in unc-101 mutants, altered axon guidance receptor localization could be responsible for the observed defects. Pry-1 encodes the C. elegans axin homolog (28).…”

Section: Resultsmentioning

confidence: 99%

Axon guidance genes identified in a large-scale RNAi screen using the RNAi-hypersensitive Caenorhabditis elegans strain nre-1(hd20) lin-15b(hd126 )

Schmitz

Kinge

Hutter

2007

117

115

“…The fact that another membranebound ciliary protein, ACIII, localizes normally in Bbs2 Ϫ/Ϫ and Bbs4 Ϫ/Ϫ neurons indicates that the BBS proteins mediate ciliary localization of distinct signaling proteins. Several studies have found that defects in ciliary protein transport can be associated with mislocalization of some signaling proteins but not others, suggesting that there are multiple mechanisms for trafficking proteins to the cilium (22)(23)(24)(25). Interestingly, hypomorphic mutations in CEP290/NPHP6, which are known to cause the early onset retinopathy Leber congenital amaurosis (26), are associated with defective ciliary localization of olfactory G proteins but not other olfactory signaling proteins, including odorant GPCRs and ACIII (25).…”

Section: Discussionmentioning

confidence: 99%

Bardet–Biedl syndrome proteins are required for the localization of G protein-coupled receptors to primary cilia

Berbari¹,

Lewis²,

Bishop

et al. 2008

425

423

Primary cilia are ubiquitous cellular appendages that provide important yet not well understood sensory and signaling functions. Ciliary dysfunction underlies numerous human genetic disorders. However, the precise defects in cilia function and the basis of disease pathophysiology remain unclear. Here, we report that the proteins disrupted in the human ciliary disorder Bardet-Biedl syndrome (BBS) are required for the localization of G proteincoupled receptors to primary cilia on central neurons. We demonstrate a lack of ciliary localization of somatostatin receptor type 3 (Sstr3) and melanin-concentrating hormone receptor 1 (Mchr1) in neurons from mice lacking the Bbs2 or Bbs4 gene. Because Mchr1 is involved in the regulation of feeding behavior and BBS is associated with hyperphagia-induced obesity, our results suggest that altered signaling caused by mislocalization of ciliary signaling proteins underlies the BBS phenotypes. Our results also provide a potential molecular mechanism to link cilia defects with obesity.melanin-concentrating hormone receptor 1 ͉ neuronal cilia ͉ obesity ͉ somatostatin receptor 3 ͉ type III adenylyl cyclase

“…In addition, AP-1 is involved in other trafficking pathways; in olfactory cilia of Caenorhabditis elegans and mammalian epithelial cells, it plays a role at the TGN or recycling endosomes in polar trafficking of certain plasma-membrane proteins to specific sites (5,6), and in fission yeast, it is involved in secretion and cytokinesis (7). Besides anterograde transport, mammalian AP-1 complex is involved in retrograde transport of the mannose-6-phosphate receptor from endosomes to the TGN (8).…”

mentioning

confidence: 99%

Arabidopsis μ-adaptin subunit AP1M of adaptor protein complex 1 mediates late secretory and vacuolar traffic and is required for growth

Park¹,

Song

Reichardt³

et al. 2013

132

168

Adaptor protein (AP) complexes are the predominant coat proteins of membrane vesicles in post-Golgi trafficking of mammalian cells. Each AP complex contains a specific medium subunit, μ-adaptin, that selects cargo proteins bearing sequence-specific sorting motifs. Much less is known about the AP complexes and their μ subunits in plants. In contrast, the μ subunit of neither the post-Golgi trafficking AP-1 complex nor the endocytic AP-2 complex has been identified. Here, we report the functional analysis of redundant AP-1 μ-adaptins AP1M1 (also known as muB1) and AP1M2 (also known as muB2). Coimmunoprecipitation revealed that both AP1M2 and its less strongly expressed isoform AP1M1 are complexed with the large subunit γ-adaptin of AP-1. In addition, AP1M2 was localized at or near the trans-Golgi network. Knockout mutations of AP1M2 impaired pollen function and arrested plant growth whereas the ap1m1 ap1m2 double mutant was nearly pollen-lethal. At the cellular level, the absence of AP1M2 entailed inhibition of multiple trafficking pathways from the trans-Golgi network to the vacuole and to the plasma membrane in interphase and to the plane of cell division in cytokinesis. Thus, AP-1 is crucial in post-Golgi trafficking in plant cells and required for cell division and plant growth.adaptor complex 1 | membrane traffic | secretory pathway | development